Diode lasers have been widely used as pump sources in solid-state laser applications. They are efficient, compact, and available at several red and near-IR wavelengths. However there are difficulties associated with adopting diode lasers for specific applications. In particular, most high-power commercial pump diodes are of the edge-emitting type, and have a rectangular waveguide with rather high aspect ratio; these diodes therefore emit highly divergent output beams with elliptical cross-sections and considerable associated astigmatism. The ellipticity and astigmatism of the pump beam can compromise pumping efficiency, especially in applications where a circular-symmetric output laser beam with an approximately Gaussian intensity profile is of importance.
Therefore in most diode-pumped solid state laser applications, various beam-correcting lens elements are conventionally used for coupling a diode laser beam into a laser gain medium in order to reduce or correct the astigmatism and convert the diode's elliptical output beam into a collimated or focused beam having a circular cross-section. These lens elements typically include at least one of a GRIN lens, an aspheric lens, anamorphic prisms pair, micro-cylinder lens, single-mode fiber etc. Although using such coupling optics does allow better matching of the pumped volume with the fundamental laser mode, it requires additional alignment steps which complicate the manufacturing process and increase laser cost. It would be therefore advantageous to provide a diode-pumped laser that does not employ lens elements to shape the pump beam and couple it into the gain medium.
U.S. Pat. No. 4,847,851 to Dixon discloses a single transverse mode diode pumped laser wherein a semiconductor pump laser is butt-coupled to an input facet of the laser gain medium so that they are spaced by “less than 0.001″”; the gain material has small, less than 500 um, absorption length and can be bonded to the output facet of the diode pump by means of an index-matching optical cement. If the pump laser has a relatively narrow waveguide so that the pump beam emitting aperture is small enough, this arrangement may provide a sufficiently small pumped mode volume to support a single transverse mode and to enable a single-spatial mode operation.
However, the output laser efficiency may suffer in such a configuration due to the elliptical crossection of the pumped region in the gain material, which does not match well with a fundamental laser mode having circular symmetry. The butt-coupled pumping arrangement may also limit the emitting aperture range and the aspect ratio of the pump laser diodes that can be used for single-mode operation of the diode-pumped laser; high-power laser diodes having wide and thin emitting aperture butt-coupled to a laser crystal may excite a sufficiently wide mode-volume to cause higher-order spatial modes to appear in the laser output, or to lead to a non-circular laser output beam. Another drawback of the butt-coupled pumping arrangement is an associated requirement to bring the pump diode laser and the laser crystal in a very close proximity during the laser assembly, with an associated risk of damaging the laser diode's output facet and reducing yield of good lasers in manufacturing.
An object of this invention is to provide a diode-pumped laser apparatus having optimized lasing efficiency in the absence of lens elements between the gain element and the pump diode.
Another object of this invention is to provide a diode-pumped laser apparatus emitting a fundamental laser mode with a substantially circular symmetry in the absence of lens elements between the gain element and the pump diode.